1. Field of the Invention
This invention relates to a method for forming a surface-roughened conductive film adapted for use in semiconductor devices. More particularly, the invention relates to a method for forming a surface-roughened conductive film used for storage nodes for capacitance storage of semiconductor memories.
2. Background Art
A surface-roughened conductive film in semiconductor devices is typically used for formation of storage nodes for forming capacitors in semiconductor memories. A method of formation of the storage nodes is described below.
FIGS. 14(a) to 14(c) show a process of forming storage nodes according to a conventional method. As shown in FIG. 14(a), a doped amorphous silicon conductive film is formed on a silicon oxide film 1 on a semiconductor wafer, and then the doped amorphous silicon conductive film is etched to form storage nodes 2. Subsequently, as shown in FIG. 14(b), a surface roughening treatment is carried out. Roughened grains are likely to be formed on the oxide film 1, but are not likely to be well formed on the storage nodes 2. As shown in FIG. 14(c), the roughened grains on the oxide film 1 are etched back and removed. At this time, the roughened grains on the storage nodes 2 are also etched back. Therefore, it is difficult to form a satisfactorily roughened surface on the storage node 2.
FIG. 15 is a flowchart showing a conventional process for forming storage nodes. Conventionally, in order to form storage nodes of semiconductor memories, a clean surface is provided after removing a natural oxide film formed by exposure to, for example, air, subsequent to formation of amorphous silicon. (See, for example, Bulletins of the 38th Meeting of the Society of Applied Physics (Spring of 1991), 31a-T4, 5, or Japanese Laid Open Patent Application No. 4-252018, and the like).
A known process of forming storage nodes is explained with reference to FIG. 15. In step 11, a conductive film made of amorphous silicon is etched to form storage nodes, followed by RCA cleaning in step 12 and HF cleaning in step 13. HF cleaning is effected such that a natural oxide film, having been formed on the surface of the storage nodes formed of amorphous silicon, is removed by means of an aqueous hydrofluoric acid. Hence, the terminated surface is provided with hydrogen to a degree as much as possible, suppressing the adsorption of oxygen. Immediately after the HF cleaning, a surface roughening treatment is initiated in step 15. This roughening treatment includes forming a nucleating film by use of a disilane gas and then subjecting to annealing treatment, thereby forming a roughened surface. Subsequently, the roughened nucleating film formed on the oxide film, which isolates storage nodes therewith, is etched back, forming separated, surface-roughened storage nodes in step 16.
The conventional roughening method is disadvantageous in that the surface roughness of the oxide film which isolates storage nodes are different from that of the storage nodes. Hence, the irregularities tend to be smaller on the roughened surfaces of the storage node. Accordingly, when the roughened nucleating film formed on the isolating oxide film is removed by etching back, the roughened portion on the respective storage nodes is likewise etched back. The irregularities on the roughened portion of the storage nodes are originally small, which leads to much smaller irregularities on a roughened surface. Therefore, the enlargement in surface area of the storage node caused by the surface roughening becomes smaller than expected.
The nucleating film formed by use of a disilane gas is influenced by the properties of the underlying layer, on which the film is formed. Hence, surface roughening may not be wholly accomplished. Thus, the stable formation of a roughened surface is not satisfactory.